Selection of optimal channel for rate determination

ABSTRACT

According to at least one example, an ambulatory medical device is provided. The device includes a plurality of electrodes disposed at spaced apart positions about a patient&#39;s body and a control unit. The control unit includes a sensor interface, a memory and a processor. The sensor interface is coupled to the plurality of electrodes and configured to receive a first ECG signal from a first pairing of the plurality of electrodes and to receive a second ECG signal from a second pairing of the plurality of electrodes. The memory stores information indicating a preferred pairing, the preferred pairing being either the first pairing or the second pairing. The processor is coupled to the sensor interface and the memory and is configured to resolve conflicts between interpretations of first ECG signal and the second ECG signal in favor of the preferred pairing.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §120 as acontinuation of U.S. application Ser. No. 14/536,189, titled “SELECTIONOF OPTIMAL CHANNEL FOR RATE DETERMINATION,” filed on Nov. 7, 2014, whichclaims the benefit under 35 U.S.C. §120 as a continuation of U.S.application Ser. No. 13/428,329, titled “SELECTION OF OPTIMAL CHANNELFOR RATE DETERMINATION,” filed on Mar. 23, 2012, which claims priorityunder 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No.61/467,599, titled “SELECTION OF OPTIMAL CHANNEL FOR RATEDETERMINATION,” filed on Mar. 25, 2011, all of which are herebyincorporated herein by reference in their entireties for all purposes.

BACKGROUND

1. Technical Field

Examples disclosed herein relate generally to the detection of cardiacfunction in a patient, and more particularly to the detection of cardiacfunction and the treatment of cardiac conditions in an ambulatorymedical device, such as a wearable defibrillator.

2. Discussion

With a wearable defibrillator worn by an ambulatory patient, thepatient's electrocardiogram (ECG) signal is obtained from body surfaceelectrodes. Determining the true characteristics of an ambulatorypatient's cardiac cycle based on an ECG signal in this manner can bedifficult. Electrical noise and electrode fall-off frequently degradethe quality of the ECG signal. In addition, the characteristics of ECGsignals vary from patient to patient due to factors such as thepatient's state of health, individual physiology, and electrodepositions on the body surface.

Known ambulatory wearable defibrillators, such as the LifeVest® WearableCardioverter Defibrillator available from Zoll Medical Corporation ofChelmsford, Mass., use four ECG sensing electrodes in a dual-channelconfiguration. That is, an electrical signal provided by one of the fourECG sensing electrodes is paired with the electrical signal provided byanother of the four ECG sensing electrodes to form a channel. Thisarrangement of ECG sensing electrodes is usually suitable because inmost cases it is rare that noise or electrode movement affects theentire body circumference. The dual-channel configuration providesredundancy and allows the system to operate on a single channel ifnecessary. Because signal quality also varies from patient to patient,having two channels provides the opportunity to have improved signalpickup, since the ECG sensing electrodes are located in different bodypositions.

SUMMARY

Examples disclosed herein are directed to a wearable medical device thatmonitors ECG signals received on a plurality of channels and interpretsthe ECG signals according to a set of preferences. This set ofpreferences indicates which channels and detection methods are morelikely to provide accurate results for the patient wearing the wearablemedical device. By interpreting ECG signals according to the set ofpreferences, the wearable medical device decreases the frequency offalsely detected cardiac malfunctions.

According to one example, an ambulatory medical device is provided. Thedevice includes a plurality of electrodes disposed at spaced apartpositions about a patient's body and a control unit. The control unitincludes a sensor interface coupled to the plurality of electrodes andconfigured to receive a first ECG signal from a first pairing of theplurality of electrodes and to receive a second ECG signal from a secondpairing of the plurality of electrodes, a memory storing informationindicating a preferred pairing, the preferred pairing being either thefirst pairing or the second pairing and a processor coupled to thesensor interface and the memory and configured to resolve conflictsbetween interpretations of first ECG signal and the second ECG signal infavor of the preferred pairing. In some examples, the first pairingincludes electrodes that are distinct from the electrodes included inthe second pairing. In other examples, one of the plurality ofelectrodes is common between the first and second pairings. The devicemay include a plurality of electrodes that are integrated into a garmentthat is worn about a patient's body. In addition, the interpretations ofthe ECG signals may detect heartbeats.

According to another example, a method of monitoring ECG signals isprovided. In some examples, the method is executed by an ambulatorymedical device as described herein. The method includes acts ofdetermining a first interpretation of a first ECG signal provided by afirst channel of the plurality of channels, determining a secondinterpretation of a second ECG signal provided by a second channel ofthe plurality of channels, determining which one of the first channeland the second channel provides a more reliable ECG signal and resolvinga conflict between the first interpretation and the secondinterpretation based upon which of the first channel and the secondchannel is determined to provide the more reliable ECG signal.

According to at least one example, an ambulatory medical device isprovided. The device includes a plurality of electrodes disposed atspaced apart positions about a patient's body and a control unit. Thecontrol unit includes a sensor interface, a memory and a processor. Thesensor interface is coupled to the plurality of electrodes andconfigured to receive a first ECG signal from a first pairing of theplurality of electrodes and to receive a second ECG signal from a secondpairing of the plurality of electrodes. The memory stores informationindicating a preferred pairing, the preferred pairing being either thefirst pairing or the second pairing. The processor is coupled to thesensor interface and the memory and is configured to resolve conflictsbetween interpretations of first ECG signal and the second ECG signal infavor of the preferred pairing.

The device may further comprise a garment that is configured to be wornabout the patient's body. The plurality of electrodes may be integratedinto the garment. The plurality of electrodes may include adhesiveelectrodes. In some examples, the interpretations of the ECG signals maydetect heartbeats.

In the device, the information indicating the preferred pairing mayinclude information indicating a first heart rate detection methodpreferred for the first pairing and a second heart rate detection methodpreferred for the second pairing. The processor may be furtherconfigured to interpret the first ECG signal using the first heart ratedetection method and interpret the second ECG signal using the secondheart rate detection method.

In the device, the processor may be further configured to determine afirst confidence level for the first pairing, determine a secondconfidence level for the second pairing, determine the preferred pairingwith reference to the first confidence level and the second confidencelevel and store the information indicating the preferred pairing in thememory. In addition, the processor may be configured to determine thefirst confidence level by comparing the first ECG signal to benchmarkinformation. The benchmark information may reflect a particularpatient's normal rhythm. In addition, the processor may be configured tocompare the first ECG signal to the benchmark information by comparing amorphology of the first ECG signal to a morphology indicated within thebenchmark information.

According to another example, a method of monitoring ECG signals usingan ambulatory medical device is provided. The method includes acts ofreceiving, by the ambulatory medical device, a first ECG signal from afirst pairing of a plurality of electrodes, receiving a second ECGsignal from a second pairing of the plurality of electrodes andresolving conflicts between interpretations of first ECG signal and thesecond ECG signal in favor of a preferred pairing being either the firstparing or the second pairing.

In the method, the act of resolving the conflicts between theinterpretations may include resolving conflicts between interpretationsthat detect heartbeats. The method may further comprise acts ofinterpreting the first ECG signal using a first heart rate detectionmethod and interpreting the second ECG signal using a second heart ratedetection method. The first heart rate detection method may be differentfrom the second heart rate detection method.

The method may further comprise acts of determining a first confidencelevel for the first pairing, determining a second confidence level forthe second pairing and determining the preferred pairing with referenceto the first confidence level and the second confidence level. In themethod, the act of determining the first confidence level may include anact of comparing the first ECG signal to benchmark information. The actof comparing the first ECG signal to the benchmark information mayinclude comparing the first ECG signal to benchmark information thatreflects a particular patient's normal rhythm. In addition, the act ofcomparing the first ECG signal to the benchmark information may includecomparing a morphology of the first ECG signal to a morphology indicatedwithin the benchmark information.

In another example, a non-transitory computer readable medium havingstored thereon sequences of instruction for monitoring ECG signals isprovided. The instructions instruct at least one processor to receive afirst ECG signal from a first pairing of the plurality of electrodes,receive a second ECG signal from a second pairing of the plurality ofelectrodes and resolve conflicts between interpretations of first ECGsignal and the second ECG signal in favor of a preferred pairing beingeither the first pairing or the second pairing.

Furthermore, the instructions may further instruct the at least oneprocessor to determine a first confidence level for the first pairing,determine a second confidence level for the second pairing and determinethe preferred pairing with reference to the first confidence level andthe second confidence level. The instructions that instruct the at leastone processor to determine the first confidence level may includeinstructions that instruct the at least one processor to compare thefirst ECG signal to benchmark information. The instructions thatinstruct the at least one processor to compare the first ECG signal tothe benchmark information may include instructions that instruct the atleast one processor to compare a morphology of the first ECG signal to amorphology indicated within the benchmark information.

Still other aspects, examples, and advantages of these exemplary aspectsand examples, are discussed in detail below. Moreover, it is to beunderstood that both the foregoing information and the followingdetailed description are merely illustrative examples of variousaspects, and are intended to provide an overview or framework forunderstanding the nature and character of the claimed subject matter.References to “an example,” “some examples,” “an alternate example,”“various examples,” “one example,” “at least one example,” “this andother examples” or the like are not necessarily mutually exclusive andare intended to indicate that a particular feature, structure, orcharacteristic described in connection with the example may be includedin that example and other examples. The appearances of such terms hereinare not necessarily all referring to the same example.

Furthermore, in the event of inconsistent usages of terms between thisdocument and documents incorporate herein by reference, the term usagein the incorporated references is supplementary to that of thisdocument; for irreconcilable inconsistencies, the term usage in thisdocument controls. In addition, the accompanying drawings are includedto provide illustration and a further understanding of the variousaspects and examples, and are incorporated in and constitute a part ofthis specification. The drawings, together with the remainder of thespecification, serve to explain principles and operations of thedescribed and claimed aspects and examples.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, components that are identical or nearly identical may berepresented by a like numeral. For purposes of clarity, not everycomponent is labeled in every drawing. In the drawings:

FIG. 1 is a functional schematic diagram of an exemplary control unitused to control electrode systems;

FIG. 2 is a flow chart of an exemplary process for configuring ECGsignal processing preferences;

FIG. 3 is a schematic diagram of an ECG signal produced by a normalcardiac cycle;

FIG. 4 is a flow chart of an exemplary process for detecting cardiacfunction from a portion of an ECG signal;

FIG. 5 is a flow chart of an exemplary process for interpreting ECGsignals according to a set of ECG signal processing preferences; and

FIG. 6 is an exemplary ECG signal produced by an idiosyncratic cardiaccycle.

DETAILED DESCRIPTION

Examples disclosed herein manifest an appreciation that some patientsregularly produce ECG signals that are highly idiosyncratic. In theseinstances, the electrical signals generated during these patient'scardiac cycles can cause conventional heart rate detection methods todetect an erroneous number of heartbeats over a given period of time.These errors are particularly problematic to a wearable medical device,such as a wearable defibrillator, because, within this context, areal-time determination of heart rate can be matter of life and deathimportance. For this reason, wearable medical devices that monitor heartrate tend to interpret ECG signals in a conservative light and err onthe side of reporting potential arrhythmias or other cardiac malfunctionto a patient, physician or others. An unfortunate side-effect of thisapproach is that, in some instances, patients are forced to regularlydeal with falsely indicated cardiac malfunction.

The conventional heart rate detection methods mentioned above includederivative-based QRS detectors that detect heartbeats by identifying QRScomplexes, which are representative of ventricular depolarization,within an ECG signal. Conventional derivative-based QRS detectorsidentify QRS complexes by comparing a slope of the ECG signal toestablished thresholds. For example, a conventional derivative-based QRSdetector may compare a magnitude of the slope of the R wave portion ofthe QRS complex to a threshold, and if the magnitude exceeds thethreshold, the QRS detector may indicate the occurrence of a heartbeat.Typically, such derivative-based QRS detectors are configured (viahardware or software) to the morphology of the heart, and the patient.

For instance, to prevent over counting, the QRS detector may beconfigured to detect an occurrence of a QRS complex only after a minimumamount of time has transpired after a prior QRS complex. In someexamples this minimum amount of time, which is referred to herein as the“refractory period” or the “programmed refractory period,” is typicallyconfigured to be about 200 milliseconds. Moreover, the QRS detector mayalso be configured to use an adaptive threshold that is based upon priorsamples of the patient's ECG signal, to account for variations from onepatient to another, or to changes in the patient's medical condition.Upon identifying a QRS complex in this manner, conventionalderivative-based QRS detectors indicate the occurrence of a heartbeat.

Given their reliance on slope, conventional derivative-based QRSdetectors may double count heartbeats where a patient's normal rhythmpresents an elongated QRS complex or an abnormally sharp T wave. Forexample, if the QRS complex generated by the patient's normal rhythm hasa duration that exceeds the configured refractory period of the QRSdetector, the QRS detector may detect a heartbeat at the beginning ofthe QRS complex and, after the refractory period has expired, detectanother heartbeat near the end of the QRS complex. In another example,if the amplitude of the ECG signal is small, the adaptive thresholdsused by the QRS detector may adjust to low values. In this situation,even a moderate spike in one of the other waves, such as the T wave,that occurs after expiration of the refractory period may result in aslope steep enough to cause the QRS detector to indicate the occurrenceof a false heartbeat.

To address these difficulties, some exemplary wearable medical devicesdisclosed herein process ECG signals from multiple channels according toa set of preference information that is tailored to fit thecharacteristics of the patient wearing the wearable medical device. Theset of preference information may include, among other information,information indicating preferred channels and rate detection methods forthe patient. For instance, in at least one example, a wearable medicaldevice processes ECG signals received via two channels, namely a frontto back channel and a side to side channel, using one or more QRS ratedetectors, which may include, for example, one or more conventional QRSrate detectors. According to this example, the wearable medical deviceresolves conflicts between these two channels in favor of a previouslyidentified, preferred channel. Further, in some examples, the set ofpreference information is automatically configured by the wearablemedical device and continuously adjusted during its operation. Forexample, the set of preference information may be adjusted based on thecurrent health of the patient, the activity of the patient, and thepresent locations of the electrodes relative to the patient's body.

The examples of the processes and apparatuses described herein are notlimited in application to the details of construction and thearrangement of components set forth in the following description orillustrated in the accompanying drawings. The methods and apparatusesare capable of implementation in other examples and of being practicedor of being carried out in various ways. Examples of specificimplementations are provided herein for illustrative purposes only andare not intended to be limiting. In particular, acts, elements andfeatures discussed in connection with any one or more examples are notintended to be excluded from a similar role in any other examples.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. Any references toexamples or elements or acts of the systems and methods herein referredto in the singular may also embrace examples including a plurality ofthese elements, and any references in plural to any example or elementor act herein may also embrace examples including only a single element.References in the singular or plural form are not intended to limit thepresently disclosed systems or methods, their components, acts, orelements. The use herein of “including,” “comprising,” “having,”“containing,” “involving,” and variations thereof is meant to encompassthe items listed thereafter and equivalents thereof as well asadditional items. References to “or” may be construed as inclusive sothat any terms described using “or” may indicate any of a single, morethan one, and all of the described terms.

Electrode System

U.S. Pat. No. 8,706,215, titled “WEARABLE AMBULATORY MEDICAL DEVICE WITHMULTIPLE SENSING ELECTRODES,” issued Apr. 22, 2014 (hereinafter the“'215 patent”), which is incorporated by reference herein in itsentirety, describes an apparatus and method for processing ECG signalsusing one or more channels. With reference to FIGS. 1A and 1B, the '215patent discloses an electrode system 100 that incorporates a controlunit 30. As described with reference to FIG. 4 in the '215 patent, atleast one example of the control unit 30 includes a processor 410, datastorage 412, a sensor interface 414, a therapy delivery interface 416, auser interface 418 and a battery 420.

Some of the examples disclosed herein for processing ECG signalsaccording to a set of preferences are implemented using the electrodesystem 100 disclosed in the '215 patent. FIG. 1 illustrates the controlunit 30 of one such example. As shown in FIG. 1, the control unit 30includes two additional components: a cardiac function analyzer 102 andpreference information 104.

The cardiac function analyzer 102 is configured to analyze a portion ofthe ECG signal to configure preference information, such as thepreference information 104, and to determine cardiac functions of thepatient using the preference information. The cardiac function analyzer102 may be implemented using hardware or a combination of hardware andsoftware. For instance, in one example, the cardiac function analyzer102 is implemented as a software component that is stored within thedata storage 412 and executed by the processor 410. In this example, theinstructions included in the cardiac function analyzer 102 program theprocessor 410 to configure preference information and determine theheart rate of the patient using the preference information. In otherexamples, the cardiac function analyzer 102 may be anapplication-specific integrated circuit (ASIC) that is coupled to thesensor interface 414 and tailored to determine the heart rate of thepatient. Thus, examples of the cardiac function analyzer 102 are notlimited to a particular hardware or software implementation. Inaddition, particular examples of the processes that the cardiac functionanalyzer 102 is configured to perform are discussed further below withreference to FIGS. 2-6.

The preference information 104 includes data that describes parametersused by the cardiac function analyzer 102 during its analysis of ECGsignals. More particularly, according to the illustrated example, thepreference information 104 identifies one or more channels from whichthe cardiac function analyzer 102 receives ECG signals. In this example,the preference information 104 also includes information that ranks eachchannel relative to the other identified channels. As is discussedfurther below, the cardiac function analyzer 102 uses this rankinginformation to resolve conflicts generated from the interpretation oftwo or more ECG signals received via the identified channels.

In some examples, the preference information 104 also includesinformation indicating one or more preferred rate detectors for eachchannel. In these examples, the cardiac function analyzer 102 uses thepreferred rate detectors of each channel to interpret the ECG signalreceived from each channel. In other examples, the preferenceinformation 104 includes an association that groups channel and ratedetector preference information into individual profiles. In theseexamples, the preference information 104 also includes data thatindicates an active profile that is used by the cardiac functionanalyzer 102 to analyze ECG signals. These examples provide the addedadvantage of easily configuring channel and rate detector informationpreferences simply by changing the indicator of the active profile fromthe active profile to another profile.

The preference information 104 may be stored in any logical constructioncapable of storing information on a computer readable medium including,among other structures, flat files, indexed files, hierarchicaldatabases, relational databases or object oriented databases. Inaddition, various examples organize the preference information 104 intoparticularized and, in some cases, unique structures to perform thefunctions disclosed herein. In these examples, the data structures aresized and arranged to store values for particular types of data.

In some examples, the components disclosed herein, such as the cardiacfunction analyzer 102, may read parameters that affect the functionsperformed by the components. These parameters may be physically storedin any form of suitable memory including volatile memory, such as RAM,or nonvolatile memory, such as a magnetic hard drive. In addition, theparameters may be logically stored in a propriety data structure, suchas a database or file defined by a user mode application, or in acommonly shared data structure, such as an application registry that isdefined by an operating system. In addition, some examples provide forboth system and user interfaces, as may be implemented using the userinterface 418, that allow external entities to modify the parameters andthereby configure the behavior of the components.

Preferential ECG Signal Processing

FIG. 2 illustrates a process 200 for configuring the preferenceinformation 104 according to one example. As shown, the process 200includes acts of receiving an ECG signal from each channel, determininga confidence level for each channel and establishing preferences. Theprocess 200 begins at 202.

In act 204, an electrode system, such as the electrode system 100discussed above, receives one or more ECG signals via one or morechannels. According to one example, the electrode system receives ECGsignals from a front-to-back channel and from a side-to-side channel.The front-to-back channel includes an electrode positioned on the chestof the patient and another electrode positioned on the back of thepatient. The side-to-side channel includes an electrode positioned onthe left side of the chest and another electrode positioned on the rightside of the patient. Other examples may employ additional, or fewer,electrodes or electrodes located in other positions on the patient'sbody. In addition, act 204 may include a variety of acts designed torender an accurate interpretation of the ECG signal. For instance, inone example, a control unit, such as the control unit 30 discussed abovewith reference to FIG. 1, filters or otherwise manipulates the ECGsignal to remove or lower the effect of noise generated by sources otherthan the patient's heart, such as other muscle movement or electricaldisturbances near the patient.

In act 206, the filtered ECG signals are analyzed to determine aconfidence level for the interpretation of each filtered ECG signal.According to the illustrated example, the filtered ECG signal of eachchannel is compared to benchmark information to determine a level ofconfidence that interpretations of the filtered ECG signal are accurate.As discussed further below, this benchmark information may includebenchmark ECG signals and benchmark values that characterize attributesof a normal cardiac cycle.

The source of the benchmark information varies from example to example.For instance, according to one example, a standardized representation ofa normal sinus rhythm, such as the ECG signal illustrated in FIG. 3, isutilized as a benchmark ECG signal. In other examples, the benchmark ECGsignal includes idiosyncratic characteristics of a particular patient'snormal rhythm. In these examples, the benchmark ECG signal may bereceived by the control unit via one or more designated channels. Forinstance, in one of these examples, the control unit records a benchmarkECG signal when the electrode system is initially prescribed and fittedto a patient. In addition, according to these examples, the benchmarkECG signal may adapt over time. For instance, the cardiac functionanalyzer 102 may adjust the benchmark ECG signal to match a compositeECG signal made up of filtered ECG signals falling within a movingwindow of a predefined duration. Alternatively, the benchmark ECG signalmay be replaced by a newly received ECG signal with desirablecharacteristics such as higher amplitudes, less noise or that moreclosely match the normal rhythm of the patient.

Like the source of the benchmark ECG signal, the comparison operationused within the act 206 to determine the quality of interpretations ofthe filtered ECG signals varies between examples. In some examples, themorphology of the filtered ECG signal is compared to the morphology ofthe benchmark ECG signal. In these examples, the level of confidenceassociated with the channel that received the filtered ECG signal isdirectly related to the fit of the filtered ECG signal to the benchmarkECG signal. More particularly, according to one example, a full cardiaccycle that includes P, Q, R, S and T waves is interpreted from thefiltered signal. In this example, a deviation from the interpretedbenchmark wave sequence of P, Q, R, S and T waves, such as one or morerepeated R waves, indicates a potential double counting of a heartbeatand results in a decreased level of confidence associated with thechannel. In some examples, this comparison operation is conducted by auser, such as a physician, through visual inspection.

In another example, the heart rate detected by each combination ofchannel and heart rate detection method is compared to the actual heartrate as determined by a user, such as a physician. In this example, theheart rate detection methods used may include derivative-based QRSdetectors, spectrum analyzers or axis analyzers, as described incommonly owned U.S. Pat. No. 5,944,669 (hereinafter referred to as the“'669 patent”), which is incorporated herein by reference in itsentirety. In this example, the level of confidence associated with thechannel receiving the filtered ECG signal is inversely related to anydifference between the heart rate detected and the actual heart rate. Inaddition, a level of confidence for each combination of channel andheart rate detection method may be established using this example.

In other examples, the refractory period is automatically configured tomore closely fit the duration of detected QRS complexes. According tothese examples, a valid QRS complex is identified by matching themorphology of the benchmark ECG signal to the morphology of the filteredECG signal. In these examples, the refractory period is adjusted (inmost cases lengthened) to encompass the duration of the valid QRScomplex while the morphology of the benchmark ECG signal continues tomatch the morphology of the filtered ECG signal. In at least oneexample, an axis analyzer, such as the axis analyzer described in the'669 patent, identifies the valid QRS complex and monitors the filteredECG signal to ensure that a match between the filtered ECG signal andthe benchmark ECG is maintained. Channels with refractory periodsadjusted in this manner are associated with a high level of confidence.

In act 208, preferences are established for determining the cardiacfunction of a given patient. In one example, preference information,such as preference information 104, is stored within the data storage412. This preference information includes a ranking of channels based onthe results of the comparison performed in act 206 with channels havinga higher level of confidence being ranked above channels with a lowerlevel of confidence. In another example, the preference information alsoranks heart rate detection methods by channel, with heart rate detectionmethods having a higher level of confidence being ranked above heartrate detection methods having a lower level of confidence. In otherexamples, the ranking information may include the strength of the fitdetected between the filtered ECG signal and the benchmark ECG signal orthe output of the axis analyzer. In some of these examples, the strengthof the fit reflects any differences in timing between interpreted ECGsignal characteristics, such as QRS complexes, and interpreted benchmarkECG signal characteristics. According to a variety of examples, act 208can be repeated at periodic intervals or as requested by a user, such asa patient or a physician. The process 200 ends at 210.

Processes in accord with the process 200 configure an electrode system,such as the electrode system 100 discussed above, to include a set ofpreferences for processing ECG signals. According to some examples, theprocess 200 is conducted by an electrode system, such as the electrodesystem 100 discussed above. In these examples, the data storage 412includes the benchmark information and the cardiac function analyzer 102conducts the acts included in the process 200. In other examples, someaspects of the process 200 are conducted by users. For instance, in oneexample, a user, such as a physician, performs the act 206 bydetermining the confidence level associated with each channel and heartrate detection method and if the user notices that the amplitude orsignal to noise ratio of a particular channel is superior, theconfidence level associated with that channel is increased. Further, inthis example, the user performs the act 208 by storing the preferenceinformation in the electrode system, via a user interface, such as theuser interface 418.

According to another example, the user, such as a physician, performsthe act 208 by causing the preference information to be stored on thecontrol unit of an electrode system via a network interface. In thisexample, a processor included in the control unit of the electrodesystem, such as the processor 410 of the control unit 30, is coupled toa network interface such as the network interface 206 described in U.S.Pat. No. 8,904,214, titled “SYSTEM AND METHOD FOR CONSERVING POWER IN AMEDICAL DEVICE,” issued Dec. 2, 2014, which is incorporated by referenceherein in its entirety.

FIG. 4 illustrates an exemplary process 400 for interpreting ECG signalsusing preference information. As shown, the process 400 includes acts ofretrieving preferences, acquiring ECG signals from preferred channelsand processing the ECG signals according to the retrieved preferences.The process 400 begins at 402.

In act 404, the cardiac function analyzer 102, discussed above,retrieves preference information, such as preference information 104discussed above, from data storage. According to one example, thepreference information includes information ranking the channels throughwhich ECG signals can be received and further indicating a preferred setof ranked channels. In another example, the preference informationincludes information ranking the heart rate detection methods availablefor each ranked channel and indicating a preferred set of heart ratedetection methods.

In act 406, the cardiac function analyzer 102 acquires ECG signals fromeach of the preferred set of ranked channels. The electrode system mayperform a variety of acts designed to render an accurate interpretationof the ECG signal. For instance, in one example, a control unit, such asthe control unit 30 discussed above with reference to FIG. 1, filters orotherwise manipulates the ECG signal to remove or lower the effect ofnoise generated by sources other than the patient's heart, such as othermuscle movement or electrical disturbances near the patient.

In act 408, the cardiac function analyzer 102 processes the ECG signalsto determine a heart rate for the patient. FIG. 5 illustrates anexemplary process 500 in accord with act 408. As shown, the process 500begins at 502.

In act 504, the cardiac function analyzer 102 processes a portion of thefiltered ECG signal received by each preferred channel using one or moreof the heart rate detection methods discussed above with reference tothe process 200. For instance, the cardiac function analyzer 102 mayutilize conventional derivative-based QRS detectors, axis detectors orothers. According to a particular example discussed further below, whenthe cardiac cycle appears to be in a normal sinus rhythm, the cardiacfunction analyzer 102 determines a heart rate using both a QRS detectorand an axis detector. In this example, the axis detector indicates theoccurrence of a heartbeat when a peak in the magnitude indicated by theaxis detector corresponds to a zero phase crossing. In another example,the axis detector may concurrently compare the quality of the fit ofmultiple ECG signals coming from multiple channels to a benchmark ECGand may use the heart rate determined in this manner provided that thequality of the fit between the benchmark ECG signal and the multiple ECGsignals meets a predetermined value. In still another example, a singlechannel fit is performed using the axis detector. In these examples thatutilize an axis detector, the quality of the fit is determined bysumming errors calculated between samples of the benchmark ECG signaland one or more ECG signals over an identified period.

In act 506, if the results of these methods agree (for example, if eachmethod detects a single heartbeat from the portion of the interpretedECG signal) then the cardiac function analyzer 102 proceeds to act 510.Otherwise, the cardiac function analyzer 102 proceeds to act 508. In act508, the cardiac function analyzer 102 resolves conflicts betweenchannels and heart rate detection methods. The particular conflictresolution procedure employed varies from example to example andexamples are not limited to a particular approach to resolvingconflicting results between channels or combinations of channels andheart rate detection methods. For instance, according to one example,the cardiac function analyzer 102 resolves conflicts that involvemultiple channels in favor of the highest ranking preferred channel. Inanother example, the cardiac function analyzer 102 resolves conflictsthat involve multiple channels and heart rate detectors in favor of theresult indicated by a majority of the channel and heart rate detectorcombinations. Continuing with the particular example discussed aboveinvolving the QRS and axis detectors, conflicts between QRS detectorresults on separate channels are resolved in favor of the channel with aresult that agrees with the axis detector result.

In act 510, if the all of the interpretations, or the favoredinterpretations, of the ECG signals indicate that a heartbeat hasoccurred, the cardiac function analyzer 102 records the occurrence of aheartbeat in act 512. Otherwise, the cardiac function analyzer 102determines if an interruption in monitoring is about to commence, suchas an interruption caused by shutdown of the electrode system, in act514. If so, the cardiac function analyzer ends the process 500 at 516.Otherwise, the cardiac function analyzer 102 returns to act 504 and theprocess 500 continues.

Examples in accord with process 500 enable an electrode system to moreaccurately track patient heart rate. More accurate heart rate trackingresults in several benefits. These benefits include more accuratepatient historical information and generation of fewer falsely indicatedarrhythmias. Fewer false arrhythmias, in turn, may result in avoidanceof unnecessary alarms and delivery of therapy to a patient, therebyincreasing the runtime between charges of the electrode system andavoiding unnecessary patient discomfort.

Each of the processes disclosed herein depicts one particular sequenceof acts in a particular example. The acts included in each of theseprocesses may be performed by, or using, an electrode system speciallyconfigured as discussed herein. Although described herein in associationwith an electrode system of a wearable defibrillator such as theLIFEVEST Cardioverter defibrillator, embodiments disclosed herein may beused with any electrode system, including conventional stick-on adhesiveelectrodes, dry capacitive ECG electrodes, radio transparent electrodes,etc. Some acts are optional and, as such, may be omitted in accord withone or more examples. Additionally, the order of acts can be altered, orother acts can be added, without departing from the scope of the systemsand methods discussed herein. In addition, as discussed above, in atleast one example, the acts are performed on a particular, speciallyconfigured machine, namely an electrode system configured according tothe examples disclosed herein.

Usage Scenario

FIG. 6 illustrates two exemplary idiosyncratic ECG signals 600 and 602received by an exemplary electrode system. As shown, the ECG signal 600was acquired via a side-to-side (SS) channel and includes QRS complex604. The ECG signal 602 was acquired via a front-to-back (FB) channeland includes QRS complex 606. As illustrated, the ECG signal 600 may bedouble counted by conventional derivative-based QRS complex detectorsbecause the QRS complexes presented within the ECG signal 600, such asthe portion of the ECG signal indicated by reference number 604, areelongated and exceed the default configuration of the refractory period.Thus, conventional derivative-based QRS complex detectors may detect afirst QRS complex at the onset of the QRS complex 604 and detect asecond QRS complex near the end of the QRS complex 604 after therefractory period has expired.

According to one example, a user, such as a physician, may perform act206 of process 200 and thereby determine that the SS channel issusceptible to double counting by comparing the morphology of the ECGsignal acquired via the SS channel to an ECG signal representative to anormal sinus rhythm. Further, the user may perform act 208 byconfiguring preference information to rank the FB channel higher thanthe SS channel. After receiving this preference information and duringthe execution of process 400, the exemplary electrode system favors theFB channel over the SS channel if the two channels yield differing heartrates. This approach results in a decreased likelihood of heartbeatdouble counting because the QRS complexes displayed within the FBchannel do not extend beyond the programmed refractory period and are,therefore, less likely to be double counted.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

What is claimed is:
 1. A medical device comprising: at least one sensingelectrode configured to be disposed on a patient; a control unitcomprising: a sensor interface coupled to the at least one sensingelectrode and configured to receive at least one electrocardiogram (ECG)signal from the at least one sensing electrode; a memory storing aconfigurable parameter indicating at least one preferred heart ratedetection method; at least one processor coupled to the sensor interfaceand the memory, the least one processor being configured to implement acardiac function analyzer to process the at least one ECG signal todetermine a heart rate of the patient using two or more heart ratedetection methods, to identify a difference in the heart rate of thepatient as determined by the two or more heart rate detection methods,to retrieve the configurable parameter, and to resolve the difference infavor of the at least one preferred heart rate detection method.
 2. Themedical device of claim 1, wherein the medical device is ambulatory. 3.The medical device of claim 1, wherein the at least one preferred heartrate detection method has at least one refractory period that isdifferent from a refractory period of a heart rate detection method ofthe two or more heart rate detection methods.
 4. The medical device ofclaim 1, wherein the two or more heart rate detection methods compriseat least one of a QRS detector, a spectrum analyzer, and an axisanalyzer.
 5. The medical device of claim 1, wherein the cardiac functionanalyzer is further configured to identify automatically the at leastone preferred heart rate detection method during operation of themedical device.
 6. The medical device of claim 5, wherein the cardiacfunction analyzer is configured to identify the at least one preferredheart rate detection method by comparing at least one signalinterpretation derived from the at least one preferred heart ratedetection method to a benchmark.
 7. The medical device of claim 6,wherein the benchmark comprises a signal interpretation of at least oneof a normal cardiac rhythm and the patient's cardiac rhythm.
 8. Themedical device of claim 7, wherein the patient's cardiac rhythm isrecorded during initial prescription and fitting of the medical device.9. The medical device of claim 8, wherein the cardiac function analyzeris further configured to adjust the benchmark to match a composite ECGsignal summarizing historical ECG signals of the patient.
 10. Themedical device of claim 9, wherein the historical ECG signals fallwithin a moving window of a predefined duration.
 11. The medical deviceof claim 1, wherein the at least one sensing electrode comprises aplurality of sensing electrodes.
 12. The medical device of claim 11,wherein the cardiac function analyzer is further configured to identifyat least one of one or more pairings of the plurality of sensingelectrodes and one or more refractory periods for the at least onepreferred heart rate detection method.
 13. The medical device of claim12, wherein the cardiac function analyzer is further configured toadjust the one or more refractory periods to avoid differences due toabnormal T waves.
 14. The medical device of claim 12, wherein the memorystores a plurality of patient profiles and the cardiac function analyzeris further configured to store, within a patient profile of theplurality of patient profiles, at least one of one or more indicationsof the one or more pairings, one or more indications of the one or morerefractory periods, and the at least one preferred heart rate detectionmethod.
 15. The medical device of claim 12, further comprising a networkinterface configured to receive preference information descriptive of atleast one of the one or more pairings, the one or more refractoryperiods, and the at least one preferred heart rate detection method. 16.The medical device of claim 1, wherein the at least one preferred heartrate detection method comprises a plurality of preferred heart ratedetection methods and the cardiac function analyzer is configured toresolve the difference in favor of a highest ranking preferred heartrate detection method of the plurality of preferred heart rate detectionmethods.
 17. The medical device of claim 1, wherein the at least onepreferred heart rate detection method comprises a plurality of preferredheart rate detection method and the cardiac function analyzer isconfigured to resolve the difference in favor of a majority of theplurality of preferred heart rate detection methods.
 18. A method ofmonitoring electrocardiogram (ECG) signals using a medical devicecomprising a memory and at least one sensing electrode, the methodcomprising: storing, in the memory, a configurable parameter indicatingat least one preferred heart rate detection method of a plurality ofheart rate detection methods; receiving at least one ECG signal via theat least one sensing electrode; processing the at least one ECG signalto determine a heart rate of a patient using two or more heart ratedetection methods of the plurality of heart rate detection methods;detecting a difference in the heart rate as determined by the two ormore heart rate detection methods; and resolving the difference byretrieving the configurable parameter and resolving the difference infavor of the at least one preferred heart rate detection method.
 19. Themethod of claim 18, wherein processing the at least one ECG signalcomprises processing the at least one ECG signal using at least two of aQRS detector, a spectrum analyzer, and an axis analyzer.
 20. Anon-transitory computer readable medium storing sequences of instructionfor monitoring electrocardiogram (ECG) signals comprising instructionsconfigured to instruct at least one processor to: store a configurableparameter indicating at least one preferred heart rate detection methodof a plurality of heart rate detection methods; receive at least one ECGsignal via at least one sensing electrode; process the at least one ECGsignal to determine a heart rate of a patient using two or more heartrate detection methods of the plurality of heart rate detection methods;detect a difference in the heart rate as determined by the two or moreheart rate detection methods; and resolve the difference by retrievingthe configurable parameter and resolving the difference in favor of theat least one preferred heart rate detection method.
 21. The computerreadable medium of claim 20, wherein the instructions configured toinstruct the at least one processor to process the at least one ECGsignal comprise instructions configured to instruct the at least oneprocessor to process the at least one ECG signal using at least two of aQRS detector, a spectrum analyzer, and an axis analyzer.
 22. Anambulatory medical device comprising: a plurality of electrodes disposedabout a patient's body; and a control unit including: a sensor interfacecoupled to the plurality of electrodes and configured to receive aplurality of electrocardiogram (ECG) signals from a plurality ofpairings of the plurality of electrodes; a memory storing a configurableparameter indicating at least one preferred pairing of the plurality ofpairings; and at least one processor coupled to the sensor interface andthe memory, the least one processor implementing a cardiac functionanalyzer to process the plurality of ECG signals to detect heartbeats ofthe patient, to identify a difference in the heartbeats detected by twoor more pairings of the plurality of pairings, to retrieve theconfigurable parameter, and to resolve the difference in favor of the atleast one preferred pairing.
 23. The ambulatory medical device of claim22, wherein the cardiac function analyzer is further configured toidentify at least one of one or more detection methods and one or morerefractory periods for the at least one preferred pairing.
 24. Theambulatory medical device of claim 23, wherein the cardiac functionanalyzer is further configured to adjust the one or more refractoryperiods to avoid differences due to abnormal T waves.
 25. The ambulatorymedical device of claim 23, wherein the one or more detection methodscomprise at least one of a QRS detector, a spectrum analyzer, and anaxis analyzer.
 26. The ambulatory medical device of claim 22, whereinthe at least one preferred pairing includes a plurality of preferredpairings and the cardiac function analyzer is configured to resolve thedifference in favor of a highest ranking preferred pairing of theplurality of preferred pairings.
 27. The ambulatory medical device ofclaim 22, wherein the at least one preferred pairing includes aplurality of preferred pairings and the cardiac function analyzer isconfigured to resolve the difference in favor of a majority of theplurality of preferred pairings.
 28. An ambulatory medical devicecomprising: a plurality of electrodes disposed about a patient's body;and a control unit including: a sensor interface coupled to theplurality of electrodes and configured to receive a firstelectrocardiogram (ECG) signal from a first pairing of the plurality ofelectrodes and to receive a second ECG signal from a second pairing ofthe plurality of electrodes; a memory configured to store a configurableparameter indicating at least one preferred pairing of the first pairingand the second pairing; and at least one processor coupled to the sensorinterface and the memory and configured to process the first ECG signalto determine a first signal interpretation of a physical condition ofthe patient, to process the second ECG signal to determine a secondsignal interpretation of the physical condition of the patient, todetermine a first level of confidence in the first signalinterpretation, to determine a second level of confidence in the secondsignal interpretation, to determine that the first level of confidenceis higher than the second level of confidence, and to store an indicatorof the first pairing in the configurable parameter.
 29. The ambulatorymedical device of claim 28, wherein the cardiac function analyzer isconfigured to: determine the first level of confidence by comparing thefirst signal interpretation to a benchmark; and determine the secondlevel of confidence by comparing the first signal interpretation to thebenchmark.
 30. The ambulatory medical device of claim 29, wherein thebenchmark includes a signal interpretation of at least one of a normalcardiac rhythm and the patient's cardiac rhythm.
 31. The ambulatorymedical device of claim 30, wherein the patient's cardiac rhythm isrecorded during initial prescription and fitting of the ambulatorymedical device.
 32. The ambulatory medical device of claim 31, whereinthe cardiac function analyzer is further configured to adjust thebenchmark to match a composite ECG signal summarizing historical ECGsignals of the patient.